Rework NTP in FuelMenu to disable if no gateway

It should be simpler to be able to start using
Fuel with no gateway or DNS. Therefore, NTP
will default to disabled with no valid gateway
configured. In most deployments, except those
in automated tests where dhcp_interface option
is provided in kernel options, NTP will be
disabled until the user re-enables it after
configuring their gateway.

Change-Id: I9cc7ba5a8e64468ab32d03d2afd8b055ffd80539
Closes-Bug: #1263934